Magnetic resonance (MR) imaging is a medical imaging technique that uses an applied magnetic field and radio frequency (RF) pulses to make images (e.g., via slices) of organs and structures inside the body. During MR imaging, the magnetic field causes magnetic field vectors of protons (typically in hydrogen atoms) to align with the magnetic field. The RF pulses cause the magnetic field vectors of the protons to be displaced (e.g., rotate) relative to the magnetic field and re-align with the magnetic field. An MRI scanner picks up signals from the protons in the body that result from magnetization field vectors re-aligning with the magnetic field. The signals may then be converted into images based on the location and strength of the incoming signals.
The achievable signal-to-noise ratio (SNR) for gradient echo (GRE) MR imaging is related to, among other things, the repetition time (TR) that exists between successive RF pulses applied to the same slice of an image, the flip angle θ to which the net magnetization is rotated relative to the magnetic field, and the relaxation time (T1) for the protons to return to their equilibrium distribution. During cardiac GRE imaging, TR is typically kept as short as possible to provide a short acquisition time to collect image data. A shorter TR, however, may result in signal saturation and low SNR, particularly for fluids and tissues with long T1 time, such as water, cerebrospinal fluid or blood.